In electromagnetic shock wave systems of spherical or other geometry, efficiency depends mostly on the electromagnetic coupling from the exciting coil to the conductive membrane. Good coupling, i.e., adaptation of the membrane to the windings of the coil, is achieved in prior-art systems by overpressure from the membrane or by vacuum in the intermediate space between the coil and the membrane. The technical effort needed to build up, maintain and monitor this vacuum or overpressure is correspondingly great and it increases the manufacturing costs as well as the weight and the volume of the shock wave source.
A shock wave source of this class has been known from a first exemplary embodiment according to DE 33 12 014 A1. A second exemplary embodiment has a rubber- or gel-like transmission medium.
A shock wave source with defined focusing, in which the coil is wound from an electrically conductive wire of rectangular cross section in order to obtain a smooth coil surface with constant, short distance from the membrane and thereby to increase the efficiency of the arrangement, has been known from the Japanese publication Patent Abstracts of Japan, JP 1-317 431 (A), Sect. C-697, Mar. 6, 1990, Vol. 14, No. 118.
DE 36 34 378 A1 describes an electromagnetic shock wave source with two coils, which are arranged in parallel and congruently, and which both attract and repel each other due to current flow in the same direction or in opposite directions. One coil is relatively immobile (is rigidly connected to the housing), and the other is relatively easily movable (it is connected to a membrane). The arrangement is to make possible the generation of shock (pressure) and vacuum waves.
An electromagnetic shock wave source, which has a flexible membrane specifically covered with a plurality of small, electrically conductive plates, has been known from EP 0 298 334 A1. The shape of the shock waves can be locally influenced in a specific manner via the inertia of masses and the conductivity of the small plates, unlike in a usual, homogeneous metallic membrane.
A prior-art shock wave source is also described in, e.g., DE-PS 34 43 295. One design, according to FIG. 1 of that publication, uses as the transmission medium a liquid, e.g., water or a liquid halogenated hydrocarbon, which is under static overpressure and it presses as a result the metallic membrane against the dome-shaped coil supported in the housing via a thin insulating layer.
The other design, according to FIG. 2 of the above noted publication, uses as the transmission medium in front of the diaphragm a rubber-elastic solid, which is also under static overpressure and thus presses the membrane. This is achieved here due to the installation of the rubber-elastic body in the compressed state rather than by an external pressure source. This requires a construction which maintains the elastic solid under pressure over a large part of its surface with stable walls. Some drawbacks are to be expected in terms of weight and volume. In addition, the transmission of the shock waves by the elastic body is, in general, not so efficient as, e.g., by a liquid.